1. Field of the Invention
The invention relates to a method and an apparatus for bending a metal thin plate, and particularly to a method and an apparatus for conducting a U-shape bending process on a workpiece such as a metal shutter for a floppy disk.
A process in which a metal thin plate that has punched into a given shape is bent into a predetermined shape is employed in a wide variety of fields. An example of products obtained by such a bending process is a metal shutter for closing an opening of a cartridge of a floppy disk, a magnetooptical disk, or the like. Such a metal shutter is formed by bending a flat metal thin plate into a U-shape with a high accuracy. In this process, conventionally, dies and an apparatus having the configuration shown in FIGS. 1A to 3 are used. FIGS. 1A to 3 are schematic views showing the main portion of a bending apparatus having a conventional punch.
As shown in FIG. 2, a metal thin plate 60 is placed between a bending punch 101 and an ejection pad 103 urged by a spring 104, and the metal thin plate 60 is then pushed into a space formed between a pair of bending dies 122 having a die channel-like shape, thereby bending the metal thin plate into a U-shape.
In this process, the operation of pushing the metal thin plate 60 into the bending dies 122 is conducted by driving a shank 150 so as to lower a base 131, or so-called bottom push bending dies are used.
In this method, the bending dies 122 are of the fixed type or they are fixed to the base 131. As shown in FIG. 3, generally, the channel width W of the bending dies 122 is set to be a sum of the thickness w of the bending punch 101 and the double of the thickness t (t.times.2) of the metal thin plate 60.
In the process of the above-mentioned metal thin plate shutter, for example, in order to accurately conduct the bending while suppressing the spring back and the spring go to a level as low as possible, it is required to press the metal thin plate by a specific pressure of about 2 to 4 t/cm.sup.2 into the bending dies 122 under an adequate pressure of the ejection pad 103.
As a result of the pressing under this specific pressure, there arises a problem in that a so-called biting state where the metal thin plate 60 which has been subjected to the bending process closely sticks to the bending punch 101 having edges of an acute angle easily occurs to make the unloading difficult to do and the workpiese is deformed when it is unloaded, resulting in a defective product. This state will be described in detail with reference to FIGS. 1A and 1B.
FIG. 1A is a perspective view showing a bending punch of a prior art apparatus and the state obtained immediately after a metal shutter, which is a workpiece, is formed by bending. FIG. 1B is a section view taken at B--B and showing the upper portion of the bending punch.
As shown in FIG. 1A, the metal shutter 60 has a cutout portion 61 in the vicinity of its one end, and tab-bent portions 60a and 60b in the vicinity of the other end. The cutout portion 61 comprises a tab-bent portion 61a.
The metal shutter 60 which has been formed by the bending process is unloaded from the bending punch 101 while being slid thereon. In order to pass the tab-bent portions over the bending punch, the bending punch 101 is required to be provided with unload channels which extend to one end of the punch and open on the end.
From the view point of maintaining the strength of the bending punch 101 to a higher level, it is preferable to form the unload channels as short as possible. Accordingly, formerly, the unload channels are elongated in direction +X (toward the right of FIG. 1A) so as to open on the right end, and the metal shutter 60 is pushed at its left end to be unloaded in direction +X (toward the right of FIG. 1A). However, the metal shutter 60 strongly bites the bending punch 101 as described above, so that a large pushing force must be applied to the metal shutter. On the other hand, the metal shutter 60 has a reduced strength because the cutout portion 61 of a large size is formed in the left portion. Consequently, such a metal shutter 60 is often deformed during the unloading process, resulting in a defective product.
In order to eliminate the application of the pushing force on the portion in the side of the cutout portion 61 of the metal shutter, recently, an improved configuration is adopted as described below. In the improved configuration, the unload channels 1b and 1c are formed so as to open on the left end of the bending punch 101 as shown in FIG. 1A, and the metal shutter 60 is pushed at its right end in direction -X by an unload pusher 70.
This configuration can somewhat improve the proportion of defective products owing to deformation of the metal shutter 60 during the unloading process. However, the upper-face channel 1b and the side-face channel 1c which are unload channels of the bending punch 101 must be formed so as to extend over a long distance as illustrated, whereby the strength of the bending punch 101 is lowered. This produces another problem in that the bending punch is easily damaged and its service life is shortened.
In addition to the above, the process of producing the punch is complicated so that the production cost of the punch is increased.
Furthermore, in the prior art method, the distance between the dies and the punch is varied depending on the scatter of the thickness of a metal plate, and therefore it is actually impossible to stably apply a predetermined specific pressure to a metal plate. Consequently, the prior art method has a further problem in that also the size of the U-shape bent portion is scattered.
In order to solve these problems, Japanese Patent Publication (Kokoku) No. Hei. 4-4047 discloses an apparatus in which edges of a die portion are constructed as inclined faces. Japanese Patent Publication (Kokai) No. Hei. 3-264116 discloses a method of, at the bottom dead point of the pressing stroke, giving a compression plastic strain of 1 to 3% to the both end portions of a web of a metal plate. In the method, the compression plastic strain is produced in the inner region within one fourth of the both side ends except the bending angle rounded portion of the bottom of a die channel.
In all of the disclosed techniques, as far as dies of the fixed type are used in the same manner as the prior art, it is impossible to completely solve the problems owing to the scatter of the thickness of a metal plate. Moreover, the disclosed techniques have a drawback in that, when parts of the apparatus wear as a result of a long term use, the positional alignment of the parts is deviated so that the bending cannot correctly be conducted, resulting in that the size of the bent portion is scattered.
In a metal shutter for a magnetic disk cartridge, particularly, the bending accuracy depends on the accuracy of the edge portions of the front and back plate portions and the top plate portion. Even when the edge portions are formed with a slightly reduced accuracy, the distance between the tips of the front and back plate portions is largely deviated from the specified value. Therefore, the bending process is required to be conducted with a very high accuracy.
SUMMARY OF THE INVENTION
The invention has been conducted in order to solve the above-discussed problems of the prior art. It is an object of the invention to provide a method and an apparatus for bending a metal thin plate in which stress generated in a punch during a pressing process is locally dispersed so that the life period of the punch is improved and the occurrence rate of the phenomenon of the biting of a workpiece is reduced to improve the proportion of defective products in the unloading process, and the period of the process of forming the punch is shortened to reduce the production cost of the punch.
Furthermore, the invention has been conducted in view of the problems of the prior art. It is another object of the invention to a method and an apparatus for bending a metal thin plate in which, even when the thickness of a metal thin plate is scattered, a predetermined specific pressure can stably be applied to the metal plate by a very simple method and configuration.
The object of the invention can be accomplished by a method of bending a metal thin plate in which, under a state where the metal thin plate is sandwiched between a punch having an unload channel which elongates from one end of the punch to a position corresponding to a tab-bent portion of a workpiece, and pressing means, the metal thin plate is bent toward the punch by dies which are respectively located at the both sides of the pressing means, thereby forming the metal thin plate into a shape having a U-like section, wherein a punch in which edge portions are chamfered as curved faces or formed into an obtuse angle of 90.degree. or more (chamfered or tapered) is used as the punch, the angle of the edge portions is formed by pressing the metal thin plate toward the punch after the metal thin plate is bent along the punch, and the workpiece is unloaded in the direction along which the dimension of the unload channel formed on the punch is shortened.
Furthermore, the object of the invention can be accomplished by an apparatus for bending a metal thin plate in which the metal thin plate is sandwiched between a punch having an unload channel which elongates from one end of the punch to a position corresponding to a tab-bent portion of a workpiece, and pressing means which opposes the punch, and the metal thin plate is bent toward the punch by dies which are respectively located at the both sides of the pressing means, thereby forming the metal thin plate into a shape having a substantially U-like section, wherein edge portions of the punch are configured as chamfers of a curved face or formed into an obtuse angle of 90.degree. or more (chamfered or tapered), the dies are movable in the direction of bending the metal thin plate and also in the direction of pressing the metal thin plate to the punch, and the workpiece is unloaded in the direction along which the dimension of the unload channel formed on the punch is shortened.
The second object of the invention can be accomplished by a method of bending a metal thin plate in which, under a state where the metal thin plate is sandwiched between a punch and pressing means, the metal thin plate is bent toward the punch by dies which are respectively located at both sides of the pressing means, wherein a die holder base for holding the dies and having a double base structure in which two bases are stacked in the movement direction is set to be a state where the bases are separated from each other by an initial distance, the bases being provided with urging forces which repel each other, a pair of the dies which are disposed on the holder base in the side of the punch are urged in the direction along which the distance between the dies is shortened, and are kept to a state where the dies are engaged in the middle of the urge against the urging force, after the metal thin plate is supplied under this state between the punch and the pressing means, the die holder base is moved toward the punch, thereby bending the metal thin plate substantially along the punch, a relative movement in the direction along which the initial distance between the bases is shortened is then produced by further pressing the die holder base toward the punch, and the engagement is canceled by the relative movement to allow the dies to be pressed toward the punch, thereby forming an angle of bent edge portions of the metal thin plate.
Furthermore, the second object of the invention can be accomplished also by an apparatus for bending a metal thin plate in which the metal thin plate is sandwiched between a punch and pressing means which opposes the punch, and the metal thin plate is bent toward the punch by dies which are respectively located at the both sides of the pressing means, thereby forming the metal thin plate into a shape having a substantially U-like section, wherein a die holder base for holding the dies has a double base structure in which first and second bases are stacked in the movement direction, an urging member is disposed between the bases to exert an urging force by which the bases are caused to repel each other, and the distance between the bases is changeable within a predetermined range, each of the dies is disposed on the first base, rotatably supported by a shaft, and urged by an urging member in the direction along which the distance between working faces of the dies is shortened, the urging members being disposed between the dies and the first base, the die being engaged by a stopper arm against an urging force of the urging member in the middle of the urge, the stopper arm being fixed to the second base, a punch holder for holding the punch is provided with a stopper which can butt a part of the first holder on which the dies are held, thereby limiting the movement of the first base, and the engagement between the dies and the stopper arms is canceled when the die holder is moved toward the punch and the stopper butts the first base, thereby further pressing the metal thin plate which is bent substantially along the punch, toward the punch.